Abstract: Diurnal variation of net photosynthetic rate(Pn) of the leaves of Achyranthes bidentata and its mutual relationship with environmental factors were studied.The results showed that net photosynthetic rate of A.bidentata presented a double-peak curve,with the peaks (7.07 and 6.87μmol/(m2•s) respectively) appearing at10:00 and 14:00. There appeared phenomenon of“photosynthesis midday depression”;The diurnal variation of transpiration rate presented a one-peak curve in the leaves of A. bidentata , and the peak of transpiration rate measured at 12:00 was 2.9 mmol /(m2•s);The diurnal variation of stomatal conductance presented a slight double-peak curve in the leaves of A. bidentata ,and the peaks of stomatal conductance measured at 10:00 and 14:00 were 0.049 mmol/( m2•s),0.045mmol/( m2•s);The positive relationship between net photosynthetic rates and PAR,transpiration rate,stomatal conductance of A. bidentata was significant, The negative relationship between net photosynthetic rates and intercellar CO2 concentration,relative humidity was significant;Eco-environmental factors combined influenced phenomenon of“photosynthesis midday depression”of A. bidentata.